Coat 1 side of each bread slice with cooking spray. Place 10 bread slices, coated sides down, on a work surface. Divide cheese and prosciutto evenly among 10 bread slices. Top with remaining bread slices, coated sides up.
Heat a large nonstick skillet over medium heat. Cook panini 5 minutes on each side or until lightly browned and cheese melts, pressing with a spatula to flatten.